This invention relates to optical data transmission systems and, in particular, to transmission systems provided with integrated dual dispersion compensators that can be readily fabricated in planar waveguide form.
Optical fiber transmission systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical transmission system comprises an optical signal source and an optical fiber transmission path for carrying the optical signals. It may also include a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM or DWDM signals) comprising a plurality of distinct wavelength signal channels. In each channel information is typically transmitted as a sequence of optical pulses.
Chromatic dispersion compensators are important components of optical transmission systems. Chromatic dispersion occurs when signal components of different wavelengths are subject to different propagation delays. Such dispersion can distort a transmitted pulse and deteriorate the information content of a signal channel. Chromatic dispersion compensators (CDCs) equalize the propagation delays among the different wavelength components and maintain the quality of the transmitted information.
There are a wide variety of chromatic dispersion compensators. An early form of CDC disposes in the transmission path a length of specially fabricated dispersion compensating fiber (DCF). The DCF slows the fast wavelength components and/or speeds up the slow components. Compensation typically requires long lengths of such fiber, and the DCF is difficult to tune. DCFs cannot be easily integrated into planar waveguide form because of their great length. Another CDC uses a dispersion compensating grating (DCG). The DCG is typically a chirped Bragg grating operated in a reflection mode. The chirping is designed so that faster wavelength components travel farther in the grating before they are reflected, with the consequence that the faster components are effectively slowed. However since the Bragg gratings operate in a reflection mode, they are typically coupled to the transmission path by bulky and expensive optical circulators. DCG compensators cannot be fully integrated because they require circulators.
Particularly promising chromatic dispersion compensators have been developed which can be integrally fabricated in planar waveguide form. Such compensators are compact and relatively inexpensive. One example of such a dispersion compensator is the optical all-pass filter described in U.S. Pat. No. 6,289,151 issued to R. Kazarinov et al. on Sep. 11, 2001 and entitled xe2x80x9cAll-Pass Optical Filters,xe2x80x9d which is incorporated herein by reference. In essence, Kazarinov et al. device comprises one or more tiny optical waveguide ring resonators optically coupled to a length of transmission waveguide.
Unfortunately planar waveguides are highly birefringent. As a consequence, introducing a planar waveguide dispersion compensator along a data transmission path produces another type of dispersion referred to as polarization mode dispersion (PMD). PMD is dispersion due, not to the wavelength components of the transmitted light, but rather to the polarization components. The light pulse may be thought of as partitionable into two different orthogonal polarization directions. In a birefringent structure, the speed of one polarization component will increase over the other, gradually spreading and distorting the pulse. As a consequence there is a need for an integrated dual dispersion compensator that can compensate both chromatic dispersion and PMD.
In accordance with the invention, an optical data transmission system is provided with an integrated dual dispersion compensator that can compensate both chromatic dispersion and PMD. The compensator comprises in essence, a substrate-supported planar waveguide structure having a planar transmission waveguide coupled to an integrated chromatic dispersion compensator and to an integrated PMD compensator. The chromatic dispersion compensator is advantageously a length of waveguide coupled to an all-pass filter. The PMD compensator is advantageously a polarization separator for separating the two polarization components between two waveguide subpaths and at least one delay element coupled to at least one subpath to compensate propagation delays. The subpath beams can then be combined to an optical signal compensated for both chromatic dispersion and PMD. In a preferred embodiment the chromatic dispersion compensator and the delay element are both tunable.